1. Field of the Invention
This invention relates to a method of attaching electronic components to printed circuit board and an attachment structure thereof.
2. Description of the Related Art
Conventionally, the method of attaching connectors and other relatively large electronic components or parts to printed circuit boards by the use of a plurality of screws is used widely. When electronic components are attached to a printed circuit board using screws, the screws are ordinarily inserted from the solder surface side into through holes cut in the printed circuit board and then screwed into screw holes in electronic components mounted on the mounting surface, as taught in Japanese Laid-Open Patent Application No. Hei 9(1997)-321462, in FIG. 4.
In addition, in order to prevent these screws from coming loose and increase the strength of the connection between the printed circuit board and the electronic components, it is a common practice to lock the screws by soldering their heads at the same time the leads of the electronic components are soldered.
Aside from the above, lead-free solder has recently become widespread due to environmental concerns. Lead-free solder typically has a higher melting point than the conventional eutectic solder, so when soldering is done using lead-free solder, the printed circuit board and electronic components are heated to a temperature higher than that in the case of using conventional eutectic solder.
For this reason, when an electronic component is attached to a printed circuit board with a plurality of screws, and this electronic component is soldered with lead-free solder, differences in the coefficient of linear thermal expansion between the printed circuit board and the electronic component cause them to deform (expand) by different amounts, specifically, the amount of increase in the distance between the plurality of through holes cut in the printed circuit board is different from the amount of increase in the distance between the plurality of screw holes formed in the electronic component, thus giving rise to a shift in the position of the screw holes relative to the through holes.
At this time, the screws that are screwed into the screw holes in the electronic component move laterally within the unthreaded through holes by an amount equal to the difference in the amount of deformation of the electronic component and the printed circuit board, and thus the positions of the screws change relative to the through holes.
If the heads of the screws are soldered and the solder solidifies in this state, the screws will be fixed and locked to the printed circuit board, so even after the printed circuit board and electronic component return to room temperature and contract, the positions of the screws relative to the through holes are unchanged. Therefore, although the difference in the amount of deformation of the printed circuit board and the electronic component can be accommodated by the screw moving laterally within the through hole when expanded, this can no longer occur when contracted.
For this reason, there is a problem in that during the cooling process, the force of shrinkage of either the printed circuit board or the electronic component (whichever has the greatest coefficient of linear thermal expansion) may cause the other to warp (flex) under pressure in the direction of shrinkage.
Such problems arise fundamentally because of differences in the coefficient of linear thermal expansion between the printed circuit board and electronic component, so depending on the material used for the printed circuit board or electronic component, they could occur even when soldering is performed using the conventional eutectic solder that contains lead.
The coefficient of linear thermal expansion of a substance does not necessarily exhibit a linear change with respect to the change in temperature, but rather it may instead increase rapidly at a certain temperature. Accordingly, even below the melting point of conventional eutectic solder (roughly 180° C.), there is a possibility of large differences in the amount of thermal expansion of the electronic component and the printed circuit board depending on the material used, so there is still a risk of warp occurring in the cooling process.